We have completed the first portion of the study as originally planned. As in prior PET blood flow or metabolic studies, we saw THC-related activation of the cerebellum, a region rich in cannabinoid receptors. There were also significant decreases in rCBF in two other cannabinoid-rich regions, the hippocampus/amygdala and posterior cingulate cortex. Degree of subjective "high" and "intoxication" with THC was particularly associated with cerebellar and orbito-frontal activation, congruent with regions active on prior PET blood flow and metabolic studies (Mathew 1998, Volkow 1996). In addition, these states were correlated with decreased regional cerebral blood flow in hippocampus/amygdala and posterior cingulate. As previously suggested, cerebellar, frontal and limbic areas were active during cognitive time estimation. Oral Marinol in single clinically prescribed doses in THC-naive subjects had no effects on the series on the series of cognitive tasks, and modest effects only on time estimation. Contrary to expectation, time estimation errors were unassociated with changed cerebellar blood flow, but rather with altered regional cerebral blood flow in a variety of regions including posterior cingulate. Blood level data are still pending.